This invention relates to a method for treating wastewater that provides for advantages over methods currently known.
With increases in population and industrialization throughout society, the need for fresh water continues to grow. This makes reclamation and reuse of water desirable when economically and technically feasible. Wastewater is an excellent source of water for reclamation and reuse, but the processed water must satisfy quality standards set by government agencies, which vary depending on the desired use. For example, water standards for urban irrigation are more stringent than standards for discharge to receiving waters when the water is not directly reused. As a result of these varying standards, wastewater treatment and water reclamation have historically been as distinctly different combinations of processes.
Conventional wastewater treatment has been generally successful at treating wastewater for discharge to receiving waters. Such facilities typically employ primary treatment methods, such as settling and skimming, followed by secondary treatment methods incorporating aerobic biological treatment, such as trickling filters and activated sludge. However, conventional wastewater treatment is unable to achieve the more stringent water quality standard required for non-potable or potable reuse. In contrast, water reclamation methods successfully achieve reuse water quality standards by adding additional treatment systems onto conventional wastewater treatment trains. In this “add-on” approach, water reclamation projects receive secondary-treated effluent from conventional wastewater treatment facilities and “add-on” tertiary technologies, such as sand or membrane filtration processes, to the treatment train. This approach has been used to supplement, rather than replace, conventional treatment methods.
There are many examples today in which conventional wastewater treatment facilities provide secondary and tertiary treated effluent water reclamation projects. An example is the Water Factory 21 plant in Orange County, Calif. FIG. 1 shows the process schematic of the Water Factory 21 treatment train. Further developments with membrane processes led to the current state-of-art approach for water reclamation, illustrated in FIG. 2. In this method, the processes of lime treatment and tertiary filtration used in the Water Factory 21 plant have been replaced by microfiltration (MF). MF involves use of membranes that serve to separate particles generally in the 0.1 to 10 micron range. MF has been demonstrated to be an excellent pretreatment step to treatment using reverse osmosis (RO). RO involves use of membranes to remove particles as small as ions from a solution. There are now several reclamation facilities that are using the MF/RO approach to water reclamation from secondary-treated wastewater. Other known forms of membrane filtration include ultrafiltration (UF), used to filter particles from a solution of molecular weight greater than about 10,000 daltons, and nanofiltration (NF), used to filter particles from a solution of molecular weight greater than about 1,000 daltons. The similarity between the old generation and the new generation of water reclamation systems is that they all use the “add-on” approach; i.e., wastewater treatment to an acceptable quality, followed by water quality enhancement by adding on processes until the desired final quality is reached.
Though this conventional approach has been generally successful at achieving desired water quality, it presents a number of problems. First, the conventional approach results in a high need for land. Addition of high technology processes to existing stream results in long treatment trains that occupy large land areas. Second, the conventional approach produces high costs associated with the many wastewater treatment facilities that are needed. Also, the conventional approach requires a substantial amount of energy to operate. Finally, the conventional approach leads to production of large quantities of waste biological solids that are expensive and difficult to dispose.
In view of the above, it is apparent that improved methods are needed for treating of wastewater that provides for increased efficiency, leading to increased production of reclaimed water for lower cost. The present invention fulfills this need, and provides further related advantages.